374 MKNARU AND LAUD fCHAl'. If) 



5. Atolls 



An atoll is an annular reef, lying at or near the surface of the sea, which 

 encloses a lagoon. Some atolls are found on the continental shelves but most 

 of them are isolated structures that rise from the deep sea. Small rings, usually 

 without islets, may be less than a mile in diameter, but in the western Pacific, 

 where atolls attain their greatest development, many have a diameter of about 

 20 mi and bear numerous reef islets. Kwajalein in the INIarshall Islands (the 

 largest existing atoll) has an irregular outline and covers 840 mi'-. 



The reefs of the atoll ring are flat, pavement-like areas, large parts of which 

 may be exposed at times of low tide. They vary in width from narrow ribbons 

 to broad bulging areas more than a mile across. Their continuity is interrupted, 

 in most cases on the leeward side, by j^asses which may be as deep as the 

 lagoon. The maximum depth of lagoons, in general, is proportional to their size. 

 Atolls with a width of 20 mi have a maximum depth of about 300 ft. On the 

 seaward side of many atolls a shallow terrace is developed, beyond which the 

 slope is steep (35°) ; on the leeward side the seaward slope may be a vertical 

 chff for 200 ft or more ; at greater depths, on all sides, the slopes flatten progres- 

 sively until they become tangential with the floor of the ocean. 



Most atolls lie in areas where unidirectional winds, such as the trades, prevail 

 during most of the year and control water circulation. The reefs are complicated 

 structures that are dynamically adjusted to their environment (Munk and 

 Sargent, 1954). Detailed studies of atolls have been carried out at Funafuti in 

 the Ellice Islands (SoUas and David, 1904) and at Bikini and nearby atolls in the 

 Marshall Islands (Emery et al., 1954). The generalized descriptions given below 

 are based largely on work in these areas, with some additions from studies 

 made elsewhere. 



A. Reefs 



On the windward side of an atoll the waves engendered by the prevailing 

 wind bring a constant supply of food, nutrient salts and dissolved gases to the 

 reef edge and the richest growth of reef-building organisms, particularly corals 

 and algae, is found there. The seaward slope of the reef margin is cut by a 

 series of regularly spaced grooves at right angles to the reef front which are 

 separated by flattened spurs that appear to be developed largely by organic 

 growth. Many of the grooves are continued through the reef margin as surge 

 channels, ending on the reef flat as blow holes constricted by algal growth. 

 The higher parts of the reef margin, particularly oi)posite islands, may be an 

 algal ridge. This ridge and the channels that penetrate it form an eff'ective 

 baffle that robs the waves of destructive power and spreads the waters over 

 the reef (Fig. 5). 



The broad flats inside the seaward margin of the reef are called the reef 

 pavement and in areas where the tide range is greater than average the pave- 

 ment is widely exposed at low tide. Large parts of it consist of bare rock which 

 may be veneered with fine sediment rich in smaller Foraminifera held together 



